The present invention relates to an electron microscope apparatus for composition and strain analysis and an observation method thereof for providing guides to process check, optimization, and device design by analyzing quantitatively composition and strained structure of corpuscles located in a heterointerface or a thin film of a device formed by crystalline growth, especially a device utilizing a strained superlattice such as a high mobility transistor or a super-high-speed semiconductor laser.
For evaluating the composition distribution and strained structure in a thin film or in the vicinity of a heterointerface of a strained superlattice device, there is needed a technique capable of measuring the composition and strained structure with a high depth resolution while keeping association with the cross-sectional structure of the device. In order to satisfy this need, several measuring methods using an electron microscope have heretofore been invented. For example, in "Method for quantitatively analyzing composition by using electron microscope" described in JP-A-62-26755, composition changes in a thin film or in the vicinity of a heterointerface are measured by impinging an electron beam upon a specimen cleaved in a wedge form, obtaining a bright-field image formed by its transmitted wave, detecting an equal thickness fringe appearing on the image, and utilizing such a phenomenon that intensity distribution of the equal thickness fringe is changed according to composition. Furthermore, in a strain measuring method described in Japanese Journal of Applied Physics, Vol. 30, 1991, pp. L52-L55, strain distribution is measured as an amount of inclination of a lattice plane by using a bright-field image and such a phenomenon that intensity distribution of the equal thickness fringe is changed by a slight inclination of the lattice plane caused by lattice strain.
According to the above described composition distribution measuring method, composition distribution can be determined beforehand for an unstrained specimen on the basis of a change in intensity distribution of the equal thickness fringe. Whether there is bending or not in the lattice plane and its angle distribution can be determined, only in case where the structure factor is similar between thin films and the intensity distribution of the equal thickness fringe does not change even if the composition is changed. This method can be applied to only specimens placed under restricted conditions.
By using only the two conventional techniques mentioned above, it is impossible to measure separately the composition distribution and strain distribution in a general specimen in which the composition change and the lattice strain are mixed. Furthermore, in the above-described strain distribution measuring method, only a simple strain structure in which the tilting angle distribution of the lattice plane is constant with respect to the direction of incidence of an electron beam is considered as a strain structure. Accordingly, the above-described strain distribution measuring method cannot be adapted to analysis of a general strain structure in which the tilting angle distribution of the lattice plane changes with respect to the aforementioned direction.
Furthermore, in the above-described strain distribution measuring method, measurements are made with a bright-field image, and hence there can be obtained only information concerning the average of tilting angles of lattice planes having respective plane indexes. Accordingly, it is impossible to analyze the dependence of tilting (direction of tilting) of the lattice plane upon plane index or analyze 3-dimensional strained structure. Furthermore, in a bright-field image, a change of intensity distribution of the equal thickness fringe as a function of the inclination value of the lattice plane is determined by crystal composition. Therefore, it is difficult to improve the measurement sensitivity of the inclination angle of the lattice plane.
Furthermore, according to the above described conventional techniques, the observer measures, on a photograph, intensity distribution of an equal thickness fringe appearing on an electron microscope image of a wedge-shaped specimen, compares the intensity distribution with intensity distribution of a calculated image obtained by using the electron diffraction theory, and thus makes a quantitative analysis of composition or strain distribution. Therefore, a skilled observer is needed for image analysis. In addition, it is also difficult for persons other than the observer to understand the result of analysis.